Missile scoring systems



May 28, 1963 M. J. COHEN ETAL MISSILE SCORING SYSTEMS 5 Sheets-Sheet 1 Filed Deo. 22, 1958 -imm May 28, 1963 M. J. COHEN ETAL 3,091,463

MISSILE SCORING SYSTEMS Filed Deo. 22. 1958 3 Sheets-Sheet 2 E: E- e.. l0

SLEEVE TARGET RATER@ MIETEN@ 5 'Fla RADIO eI/RADIO 57 TRANSMITTER 50 TRANSMITTER es RADIO RECEIVER e. FLIGHT PATH MPUTER MULTI PART 53: E

NUCLEAR RADIATION J I2' DETECTOR I I I ANTI- COINCIDENCE T0 AMPLIFIER CIRCUIT T E- NUCLEAR z-E-- RADIATION P- AMPLIFIER J2 /'4 DETECTOR NUCLEAR AMPLIFIER RADIATION AMPLIFIER -n- CONTROL DETECTOR AMPLIFIER RAD'O'WAVE CONTROL DETECTOR SHOCK WAVE INVENTORS DETECTOR MARTIN J. COHEN 46 HENRY C. GIBSONI JR May 28, 1963 M. J. COHEN ETAL 3,091,463

MIssILE sooRNG SYSTEMS Filed Deo. 22, 1958 5 Sheets-Sheet 3 .:":|:E..E1A 1:15-58- E:|:E.E|C

CALIBRATION sLGNALs l5 FT. HIT SIGNAL BOFI'. HiT SIGNAL l Sec 400-2000cps IOOOcps 400cps MODULATION MODULATION MODULATION INVENToRs MARTIN J. col-IEN 531-5- HENRY c. G|BsoN,JR.

United States Patent Q 3,091,463 MISSILE SCGG SYSTEMS Martin I. Cohen, West Palm Beach, and Henry C. Gibson, Jr., Palm Beach, Fla., assignors to Franklin Systems, Inc., West Palm Beach, Fia., a corporation of Florida Filed Dee. 22, 1958, Ser. No. 781,954 22 Claims. (Cl. Ufa-102.2)

This invention relates to systems for scoring munitions, missiles, or projectiles, and more particularly to systems for determining miss-distance or firing error through the use of nuclear radiation.

Scoring systems which are based upon a visual indication of the hits of munitions directly upon a target are well known. A common system employs an airborne target sleeve that is attached to a towing aircraft by a tow line or drag line. The scoring of munitions red at the target sleeve may be determined by visual inspection. More elaborate schemes have been devised in which hits are scored by proximity of the munitions to the target. With such systems actual contact of the munitions with the target is not required. In view of the great increase in destructive capability of modern munitions and the use of proximity fuses and the like, scoring systems which depend upon proximity, rather than Contact, have assumed increasing importance. Some of the systems employed heretofore use light waves, radio waves, shock waves, or electrostatic charges as the basis of proximity determination.

The presen-t invention is based upon the use of nuclear radiation. More specically, gamma rays are used, because of their long range and high energy content. A missile scoring system employing such radiation has deiinite advantages over systems of other types. Among these advantages are the following:

(1) The radioisotope gamma ray source employed transmits radiation spontaneously and independent of ordinary environmental influences, such as temperature and pressure.

(2) The life of the source can be made as short or as long as desired. The decay of strength can be selected by radioisotope selection and can be calibrated from hours to years.

(3) The radiation is non-jammable by electronic equipment.

(4) The radiation does not interfere with other electronic equipment used in the system tests.

(5) The radiation is non-detectable outside of the design range.

(6) The system operates in an uncrowded region of the electromagnetic spectrum.

In addition, the gamma ray source is very small, is simple to associate with a missile, requires no external or internal power supply, and can be readily varied in magnitude.

Accordingly it is a principal object of the invention to provide a system of the type described having the foregoing characteristics and advantages.

Another object of the invention is to provide an accurate, lightweight missile scoring system which may readily be made airborne.

A further object of the invention is to provide a system of the foregoing type of which the etfective target. volume may be readily varied and controlled.

A still further object of the invention is to provide a system of the foregoing type in which the scoring may be made substantially independent of missile to target relative velocity over a wide range of velocities.

An additional object of the invention is to provide a system of 'the foregoing type having a plurality of predetermined target ranges.

Yet another object of the invention is to provide a system of the foregoing type in which spurious indications of hits may be substantially reduced.

Still another object of the invention is to provide a system of the foregoing type which is continuously and automatically calibrated, and in which calibration information may be transmitted to a remote monitor.

An additional object of the invention is to provide a system of the foregoing type which produces a hit indication only when the radiation detected exceeds a predetermined threshold value.

A still further object of the invention is to provide a system of the foregoing type including telemetering apparatus which transmits hit information to a remote hit indicator.

Still another object of the invention is to provide a system of the foregoing type in which the target has assoeiated with it an indicator for producing readily visible hit indications.

A further object of the invention is to provide a system of the foregoing type in which the accuracy is increased by making the system responsive to the concurrence of events.

Another object of the invention is to provide a system of the foregoing type employing a plurality of radiation detector units which are used to produce an indication of Ithe iiight path of a missile relative to a target, the nuclear radiation providing both range and distance information.

The foregoing and other objects, advantages, and features of `the invention, and the manner in which the same are accomplished will become more readily apparent upon consideration of the following detailed description of the invention in conjunction with the accompanying drawings, which illustrate preferred and exemplary embodiments of the invention, and wherein:

FIGURE l is a block diagram of a first form of the invention;

FIGURE 2 is a block diagram of a second form of the invention;

FIGURE 3 is a block diagram of a modification of the invention;

FIGURE 4 is a block diagram of another modification;

FIGURE 5 is a block diagram of still another modification;

FIGURES 6A, 6B, and 6C are graphic illustra-tions of certain signals utilized in the invention;

FIGURE 7 is a geometric diagram illustrating certain principles of the invention;

FIGURE 8 is a partly sectional View of one form of radiation detector which may be employed in the invention; and

FIGURE 9 is a partly sectional view of another form of radiation detector which may be employed in the invention.

INTRODUCTION Briefly stated, the scoring system of the invention depends upon the labeling of missiles or projectiles with a source of nuclear radiation, gamma rays. Missile scoring is determined by the proximity or miss-distance of the missile with respect to a target, and in general, the effective volume of the target is much greater than the volume of its actual physical configuration. Target volume is generally a function of the strength of the radiation source and the sensitivity of the radiation detector which forms a part of the target. In a preferred form of the invention hits are registered when the radiation detected by the radiation detector exceeds a predetermined threshold value, it being apparent that the term hit as used herein denotes passage of a missile with a predetermined proximity to the target and not necessarily an actual contact of missile and target. In accordance with one feature of the invention, hit information is transmitted to a distant indicator. Such information is also indicated visually at the target. In accordance with still another feature of the invention the system is calibrated continuously to maintain the pre` determined threshold value of sensitivity. As will also be described, steps are taken to eliminate spurious indication of hits which might be caused by background noise or the like. In another form of the invention, a plurality of radiation detectors is employed with a computer to produce both range and direction information from which the flight path of a missile with respect to the target may be determined.

A theoretical prologue will set the environment for the description of `the systems of the invention which follows. Referring to FIGURE 7, it is assumed that a spherical nuclear radiation receiver is located at O and that the diameter of the receiver is 2r, where r is the radius. The cross-sectional area, A, of such an omnidirectional receiver is A=1rr2. Assume that a munition labeled with a source of nuclear radiation having an activity of C millicuries fis traveling along a path such as one of those indicated by the arrows in FIGURE 7 and at any instant of time t is at a distance m feet from the target at O, where m is a function of t. If E is the eiciency of detection of the receiver, then the rate S in photons per second detected by the receiver is given by the relationship In a system of the invention to be described provision will be made for substantially continuous calibration of the radiation receiver, so that E may be considered a constant, independent of temperature, power supply variations, etc. The factor AE expresses the receiver performance and the term C the transmitter performance.

To obtain a truly spherical threshold hit distance pattern (threshold hit distance being the maximum distance from the target which will be registered as a hit), it is necessary to consider the maximum relative speed of target and munition. For mm. shells this value of velocity, v, is about 3,000 feet per second. For air-to-air missiles, a figure for v of 2,000 feet per second is representative. If S is measured in counts per second during a period of time T seconds in which an average of n counts is detected, then If the change in radius (mb-M1) as a result of the distance vT that the munition moves in T seconds along path ab, is small compared to M1, the threshold distance (see FIG. 7), then the threshold hit pattern is spherical to maximum relative velocity v. Consider the sphere of radius M1 feet, the threshold distance. If a munition approaches the target such that a hit is registered, the distance m from the target must be less than or equal to M1. If m is greater than M1 nothing happens.

The radioisotope labeled munition is randomly emitting 3 107 gamma photons per second, per millicurie of labeled activity, in a practically uniform omnidirectional pattern. In practice a munition will traverse a path which in the vicinity of the target is approximated by a straight line, such as ab in FIGURE 7. In the time T required for the munition to traverse the path ab relative to the rapidly moving target at O, an average number of gamma ray photons n will be registered by the receiver at the target.

The selection of T is based upon several factors. The important ones are the desired M1, the permissible error in M1, the maximum relative speed v of target and munition, and the use of minimum strength of radioisotopes, the assumption being as above that vT is very much less than M1.

Consider as an example that v=3,000 feet per second and that T=.005 second. Then path ab is .005X3,000 or 15 feet. The extremes of the path at a and b are located a distance ma and m1, from the target, this distance being approximately 16.8 feet in the example given. The total path difference is thus approximately 10%, which is 15% from the mean value. If M1 were 30 feet, the path extremes would be 31 feet from the target, which is a i1.5% change in radius. At a lower relative velocity, the signal S measured in the period T of .005 second would cover a still shorter path with a correspondingly smaller change of radius m.

Since the receiver is continually measuring the counting rate S with a time constant T, it will select automatically the maximum signal received during the missile flight on a segment of the path, such as ab, and determine whether the signal meets the criterion of a hit. On the basis of the use of a small time constant T, to measure S, the change in radius is smaller than the tolerance of the threshold hit distance M1. Y

The general signal Equation 1 may be written in integral form to show the time dependence, namely:

Tdt

To a first approximation, a simple numerical integration of Equation 3 can be performed to `obtain the average value S. Since this integrated value differs little from use of the fixed distance M1 for determination of the threshold signal, and since the statistical randomness of the radioactive signal contributes a larger uncertainty in the threshold signal, Equation 1 can be used. The same discussion applies at any threshold distance M1.

Equation 1 can now be used to determine the average quantity of radioactive material required on the munition to produce a threshold hit indication at 30 feet, for example, or at a lesser distance from the target. At 15 feet, for example, one-quarter as much activity is required, all other conditions remaining constant.

The following table gives typical values for the counting rate S in photons per second for the time constant T of .005 and .01 second, Also shown is the average number of counts received, n, in the period T.

r (it.) A (it.) m E C S (count/sec.) T n (ft.) (me.) (sec.) (counts) .20 30 0.8 1 5. 2)(10 2 .005 2. 5 M .35 30 0.8 1 9. 2X10 2 .005 4. 5 M .35 30 0.8 2 1. 8X10 3 .005 9 M .20 30 0. 8 2 10X10 3 .005 5.0 1% .35 30 0.8 0. 65 6. GX10 2 .005 3.0 .35 30 0.8 1 92 .03 2.7 M .20 15 0.8 1 2 X10 3 .005 10 K 20 15 0.8 .3 2 X10 3 .005 3 9g .35 15 0.8 1 3.7 103 .O05 18.5 $6 .35 15 0.8 .3 1.1X1O 3 .005 5.5 .35 15 0.8 .2 7.4 1(]z .005 3.7 .35 l5 0.8 .1 3. 7X10 2 .01 3. 7 1/5 .35 15 0.8 O6 2. 2X10 2 .01 2. 2

The foregoing discussion has assumed that an average signal S in counts per second and n in counts is a quantity uniquely determined by Equations 1 and 2. This is strictly true only if the number n is large. However, a determination of the achievement of the threshold signal must be made in one swift munition pass and in a short period of time T. If a minimum amount of radioisotope is used, n may be quite small. Therefore, the signal is determined by whether or not a certain number of counts q is received in the period T, where on the average the relationship expressed by Equation 2 is applied. In other words, a

single sample q must be larger than the average number n in order to register as a hit.

It is necessary that the value S produce a highly probable indication of a threshold signal at the selected dis- 6 the minimum radioisotope activity being that required for a threshold level of two or more counts per period, the spurious background hits occur once in 200 periods. 'I'his may be considered relatively frequent, but spurious tance of M1. To evaluate the statistical nature of the hits can be eliminated by use of techniques to be described random nuclear gamma. photon signal, the use of a standhereinafter. A better threshold signal may be defined aS ard statistical equation, namely Poissons relationship is one lWinch produces three or more counts per period, required. This special case of the Gaussian distribution 11 WhICh CaSe the Probablllty 0f fecordmg a SPUUOUS hlt is given by the expression: 1S Very Small.

www A PRACTICAL EMBODIMENT Pq: ql (4) FIGURE 1 illustrates a preferred practical form of the invention. In this iigure a missile 10 is shown ap- Hetreyqts the Fealflve Pobaplltly gf Sobefrng o proaching a nuclear radiation detector 12. The missile coun-shm e perm an e e a e e n 15 may have a wide range of sizes, as for example, from 20 logIlt m.' u that T e ual 005 econd from the nim. shells to large guided rockets. Each missile is proforesolrrgubaaullue of ncetven 2 nd 9 nlay be de vided with a source of nuclear radiation; typical schemes will be set forth later. The amount and hence the weight p temmed for Val-mus recelver 151265 and radlactlva 311,5 of the nuclear material which must be associated with the f Httefsrengths' examp e usllgf/[he at? gtlven m 20 missile is very small, 'a -source of from about 100 to t e 1 t ow Ol e averaged al or a ga fmg' 2 00 microcuries of equivalent gamma activity being sutlil sampcs re putles tre tecoljfe m a pgn? {e2/a1' f cient to produce a practical threshold signal, and as will tec ues ma e em o e to re uce t e activit reaverage. Each .005 second period is astatistically random quired1 The gudear 13/arial may be attached tg the Sample' Therefore a demtel Ptrlablhy forl ecelflg projectile in the form of a label secured to the tip. For attirant? @astanti me fg placid a out t ee inc es ac rom e nose. t is a simpe 5h11 above The Probabllhtyd of recllVng ODIIY none matter to providelthe necessary nuclear radiation masie alfrmgghe afgdynm gog- 07 1 2. erties o t e missie. t is pre erre t att e materia e 1t 15,21 Cerfamty that elther Somethmg or nothmg Wlu be provided such that the radiation pattern of the nuclear recelved 1t follows that: emission is substantially omnidirectional.

p0+p1+p2+p3+- +P=1 (5) The nuclear radiation detector 11g, together with the Also the probability of recording a number greater than isfscacrilattnlltlrlgeeltts tstlziolieril litria tectivdlimab form Ptard 1 air a c ne r moun e a PfeaSSgIled Value C all be defemiled- FOY examplefhe on 'a drone. It is preferably of the type having an omni- Pfebablity 0f IeCOfdlDg more Ihal'l tWO Pulsee Pel'- Period, directional radiation pattern and which produces out- WhlCh may be represented by the term P 2, 1S gwen by: put pulses in response to detected radiation. It may con- P :1 Pl P P 6 40 sist of a shell of special scintillation-activated plastic, 2 0+ 1+ 2) such as polyvinyl toluene. The shell may have an outer From Equations 4, 5, and 6 a table may be prepared diameter of about 8 and an inner diameter of about 6". which gives the probability of observing certain values A typical unit is illustrated in FIGURE 8, wherein the of pulses for a period based on the statistical average shell is of spherical configuration, being formed from two value n expected for the period. The following is such hemispherical sections 12a, 12b. Alternatively a tetraa table; hedral or other configuration might be used. A photoa b c d e f g h n P0 P1 P2 Pa P1 Pi P:

0. 1 90483 090483 .00452 1. 5x10-4 .005 .00016 00001 0. 25 .77880 19470 .02434 2. 01 X10-3 .03 .0022 .00019 0. 5 .e005 3032 0080 .013 .090 .026 013 1. 0 .36s 36s 184 .061 2e .08 019 i 2. 0 135 270 270 ,is .60 .33 15 3. 0 0497 149 224 224 s0 5s 36 4.0 .01s .072 144 .196 .91 .77 57 5. 0 .0007 .0335 084 .140 96 88 74 0. 0 00248 .0149 0446 08s 983 04 85 9. 0 .00012 .00108 .004s 015 0989 .993 .978 12.0 6.1 10 7. 3 i0i 4.4 i04 i.76 103 .999994 .99992 .9995 18.0 i 44x10-s 2.6x10-1 2. 35 i0 1.4 10i 1.00 1.00 .999908 From this table can 'be selected a suitable number of pulses q to constitute the threshold signal. Increasing the number of pulses requires a larger receiver, a stronger radioactive label, or both, for a given threshold dis- 1 tance. It is the intrinsic nature of nuclear radiation i sources that due to the randomness of emission there multiplier tube 12o (such as an RCA Type 61199, an RCA Type 5819, a DuMont Type 6364 or a DuMont Type 6292) is shown in optical contact with the scintillation material. 'Ihe sphere of material may have an outer reflecting coating such as a magnesium oxide diffuse reflector or an aluminum mirror rellector. The active porwill be a region of statistical probability from 0 to 1 that a hit will be recorded for a certain threshold level. In a practical application a value of probability of 0.5 may be used to give the threshold level.

The threshold signal, as a function of probability Pq, deterrni'nes the niunitions radioisotope activity, the accuracy of the threshold distance, and the eiect of backtions of the detector are placed in a light tight housing lZd.

The scintillation detector 12 requires a suitable power supply, as for example, one which produces 2,000 volts at about 5 inilliamperes. With a continuous calibration system to be described, a non-regulated power supply may be employed. For example the power supply may ground noise in producing suprious hit signals. With include a prime battery, a silicon transistor oscillator, a

step-up transformer, and a high voltage silicon rectifier. If the oscillator generates a high frequency, of say 100,000 cycles, an RF type transformer with a powdered iron or air core may be employed. Such a power supply will weigh very little.

During the passage of the nuclear labeled munition, gamma ray photons strike the detector y12 and are absorbed by the scintillation material. The scintillation material produces a weak fiash of light which is converted to electrons and multiplied by the photomultiplier tube to a signal level of say .0l to .1 volt. This pulse, or count, is amplified by an amplifier 14, which may be a 2- or 4 stage proportional pulse silicon transistor amplifier with a band width of 100,000 cycles, for example.

The signals from the amplifier are sorted into two amplitude categories by two pulse height selectors and pulse shapers 16 and 18. These units per se are well known. Pulse height selector 16 passes pulses of 1 rnev. or larger, while pulse height selector 1S is a differential type and passes pulses in the range of .1 to .2 meV.

The system of FIGURE 1 produces two types of hit indications, one indication for hits within a 30 foot range of the target and another for hits` within a 15 foot range. Accordingly, the large pulses from the pulse height selector 16 are channeled to a pair of integrator-pulser units 20 and 22 corresponding to the respective ranges. When the voltage reaches a prescribed threshold level in either network an output pulse is produced. For example, each of units 20 and 22 may comprise a passive RC integrator of predetermined time constant corresponding to the range and a pulser which is actuated when the integration voltage reaches a predetermined level. The pulser may include a suitable multivibrator and pulse shaping network.

Each of units 20 and 22 produces an output pulse that is characteristic of the unit. For example, each may produce a one second pulse, the pulse from unit 20 having an amplitude of volts, for example, and the pulse from unit 22 having an amplitude of 10 volts, for example. These pulses may be used to gate on and control the frequency of a variable frequency audio oscillator 24. For example, the normally inactive audio oscillator 24- may produce a one second tone of 400 cycles (see FIG. 6C) when an output pulse is applied from unit 20 and a one second tone of 1,000 cycles (see FIG. 6B) when an output is present from unit 22.

The output of the audio oscillator 24 is applied to a modulator 26 which modulates the carrier of a telemeter transmitter 2S. The carrier may be supplied by a crystal oscillator and multiplier 3.0l which produces an RF carrier in the band of 215 to 235 megacycles per second, for example. The transmitter may produce 3 to 5 watts of power and operate at a 0.1% duty cycle. It may have a lightweight transistorized power supply. Signals from the transmitter 28 are radiated and are received by remote units, such as a hit indicator 32 on an aircraft which is firing the missiles to be scored and a hit recorder 34 on the tow aircraft or the ground.

At the target, hits may be indicated visually by an indicator 36. This unit may comprise an electronic flash of say 5001 watt-seconds which will produce a brilliant hit indication. The unit may have a power supply including a battery and a storage capacitor. By virtue of the indicators provided, hits may be registered at the target, on the firing plane, on the towing aircraft, and on the ground `at a control station.

Maintenance of the threshold signal level and hence the accuracy of the system may be ensured by continuous calibration. In the preferred calibration scheme illustrated in FIGURE l, a built-in calibration source 38 is ernployed. This source may be a radioisotope such as carbon 14, which continually emits radiation below about .15 mev. maximum beta radiation. The radiation detector 12 is continually exposed to the radiation from source 33 and produces output pulses which are amplified by the `amplifier 14. The height of these pulses is much less` than the height of the pulses corresponding to radiation from the missiles 10, and hence the calibration pulses pass through the pulse selector 18, rather than the pulse selec- Vtor 16. The output of pulse height selector 18 is supplied to a calibration counter 40 which counts the calibration pulses and produces a D.C. output voltage dependent upon the counting rate. This output voltage controls a servo motor gain control 42 for the amplifier 14. It is apparent that the amplifier 14, pulse height selector 18, calibration counter 40, and servo motor gain control 42 form a closed -loop servomeclianisrn. The gain control may include a 6 volt D.C. vrnotor and a gear train which drives a low torque ten turn amplifier gain control potentiometer. The direction of rotation of the motor is determined by the value of the D.C. output voltage from the counter 40. An increased count rate increases the output voltage and drives the motor in one sense to decrease the amplifier gain. A decreased count rate decreases the voltage and drives the motor in the opposite sense to increase the amplifier gain. A narrow dead zone is provided vvhere the motor is not activated. Because the loop drift is small and the correction time rapid, the motor may operate only about .01% of the time.

In order that the state of calibration may be monitored the servo loop correction signal also tone modulates the audio oscillator 24 at about a. 0.1 duty cycle with a tone between 400 and 2,000 cycles per second (see FIG. 6A). When the system is properly calibrated, a 1,000 cycles per second tone will be generated. Deviations are indicated by a change in tone.

The calibration signals are of very short duration, while the hit signals are of one second duration, for example. For a 30 foot hit there will be an output from the integrator unit 20, but not unit 22, and `for a foot hit, there will be outputs from both units. Since the oscillator can only generate one vfundamental frequency at a time, the 15 foot signal, which produces a greater control voltage for the oscillator will mask the foot signal, but of course the indication of a 15 foot hit is of itself an indication of the passage of the missile within the 30 foot range. Being of very short duration, the calibration signals will be masked by the hit signals when hits are indicated, but the calibration signals will be transmitted when hits are not present.

Background Noise Suppression Design of the system for minimum background noise requires a thickness of scintillation material of the receiver which effectively produces pulse amplitudes proportional to gamma photon energy up to 1.0 mev. or better. In effect, the scintillation material must have Vsufficient thickness to permit the gamma ray photons to dissipate all their energy. The larger the scintillation receiver, the more probable is the occurrence of complete absorption of the gamma ray photon in the energy range larger than 1.0 meV.

scintillation detectors which are of the resonant size for the radioisotope label used on the projectile will yield maximum signal-to-noise ratio by permitting the pulse height selector to reject low energy pulses. This effect is a result of discriminating against background signals which result from naturally occurring radioactive isotopes, which yield undesirable noise at lower gamma ray energies. Such naturally occurring materials as the radioisotopes of potassium and carbon 14 contribute appreciably to background signals below 1 mev. The contribution to background signals from radium, thorium, and actinium derivatives also decreases as the energy threshold is increased. In addition, cosmic radiation contributions to background also decrease as the energy of the signal pulse threshold is increased. It has been calculated that a l thickness of scintillation material of the activated polyvinyl toluene polymer type gives sufficient signal amplitude for 1 mev. pulses and larger.

By using electronic circuitry with the scintillation detector which rejects background signals less than 1 mev., the residual background noise will be less than j/20 of a count per second per pound of scintillation detector. The principal contri-bution will then arise from the more improbable large events in the cosmic radiation rather than naturally or man-made radioisotope contamination.

With the spherical scintillation detector of the type previously described, that is, 8 outer diameter and 6" inner diameter, the weight will be approximately 3 to 5 lbs. The background signal Will then be about 1/5 of -a count per second. 'This background signal produces a probability of observing a random hit of only .00016 for three pulses per period and only 1 105 for vfour pulses per period. On the average, one spurious hit is `observed every 30 seconds in the former case, and every 540 seconds in the latter case. Therefore, even without the use of additional spurious event lor background noise suppressors, it is vpossible to get negligible spurious events in the case of four pulses per period.

To make spurious events even more negligible, certain additional suppressor techniques may be employed. One of these is illustrated in FIGURE 3, wherein the radia- .tion detector 12 is a ydevice having a plurality of separate radiation sensitive parts, each With its own photomultiplier. If these parts are isolated from one another, as by -separating a spherical detector into sectors ydefined by lead plates, separate outputs may be obtained. If a signal is received in all detectors it is probably a large cosmic ray event and can be rejected by an anti-coincidence circuit such -as is indicated at 44 in FIGURE 3. It is probable that a munition signal will be received by only one section of the spherical detector at a time, and hence the anti-coincidence circuit 44 may be used to pass output signals to the amplifier of this system, such as ampliiier 14 in FIGURE 1, only when radiation is detected by a single part of the `detector 12.

FIGURES 4 and 5 illustrate the use of concurrent event circuits for reducing the sensitivity of the system to spurious signals. In FIGURE 4 the system includes a shock wave or sound Wave detector 46, such as a simple rugged microphone type receiver. This device is actuated by the shock wave associated with the passing of a missile, andits output may be utilized to gate the amplifier 14 on only when -a shock wave is present. Thus the output of the nuclear detector 12 will be passed through the amplifier 14 only when the amplifier is gated on by the shock wave detector 46.

The system of FIGURE 5 is similar, except that a radio wave detector y48 is used in place of the shock wave detector. The radio wave detector may be actuated by an RF signal transmitted at the same time that the missile is tired or launched. This signal may be transmitted from a suitable ground station. Here again, the signals from the radiation detector 12 will be passed by the amplifier 14 only when the amplier is gated on by the output of the radio wave detector, indicating the presence of RF energy received by the detector. The detector 48 may be sharply -tuned so as to respond only to -a predetermined frequency.

It will be apparent that in both the schemes `of FIG- URE 4 and FIGURE 5, the outputs ofthe respective detectors could be applied to a coincidence circuit which would pass signals to the amplifier only upon the concurrence of the aforementioned events.

The use of the lforegoing spurious event suppression devices permits the use of a threshold signal of only two pulses or more per period. The result is a negligible spurious hit background which may be as low as one spurious hit per 100 missile hits, and no random hits will be recorded when a missile is not present.

THE NUCLEAR RADIATION SOURCE iFor the radioisotope label it is desirable to use a ma-` terial having a moderately short half-life activity in order to eliminate radioactive accumulation. However, ya very short half-life leads to practical problems of maintaining constant transmitter activity. As set forth previously, another requirement for the radioisotope is that the gamma ray energy emitted should be appreciably :higher than say 1 mev. The following table is a list of certain radioisotopes, their half-life, and the energy of the gamma rays emitted.

Element Halt-life Principal gamma rays Sodium 24 15 hours.. 2.7 mev` Iodine 131 8 days 0.34 mev., 0.64 mev. Barium 140, Lanthanum 140 12.8 days 2.3 mev., 2.6 mev.,

2.9 mev.

Antimony 124 60 days 1.7 mev., 2.1 mev. Scandium 46 85 days .90 mev., 1.12 mev. Zinc 65 250 days 1.12 mev. Silver 110 m 270 days 1.4 mev., 1.5 mev. Ruthemum 106, Rho urn 06 1 year 1.55 mev., 2.41 mev. Cobalt 60 5.3 years 1.17 mev., 1.33 mev.

`In view of the criteria expressed previously, it is apparent that antimony 124, silver 110m, and zinc 65 are suitable radioisotopes.

Several methods of attaching the radioisotope label to the munition may be employed. The required radio- .active material may be combined in a continuous foil or thin plastic as an insoluble material. Ceramic, clay, and glass type chemical compounds have recently found wide usage in insoluble binding of radioactive materials. ISuch a compound can be put into strip form, say 1/16 wide and .005 inch thick and supplied in a storage magazine. The magazine can be placed in a simple hand tool which will dispense a segment of this strip to the munition along with a high tack thermosetting adhesive. Rubber based fand epoxy based adhesives with very high instant tack strength which increases with time `and heat may be employed.

' 'The -device which attaches the radioisotope material to the shell can also select the proper amount of activity by determining the area of `foil to be attached. With antimony 124 (half-life 60 days) a foil diameter of 1/16 of an inch can be used initially. This area can be progressively increased if the originally supplied strip of foil is not used up in say 30 days. Every 10 days 10% more area is added to the foil to keep the shell activity the same. This could be done Aautomatically by suitable time control.

Other ways of associating the radioactive source with the munitions are as follows:

(1) By mixing the radioactive material as an ingredient of the shell material during manufacture of the munitions.

(2) By nuclear pile activation of the shell material.

(3) By application of an adherent plating or paint.

`(4) By inserting an ac-tive core in the munitions.

With Va 15 foot threshold scoring distance for `an omnidirectional radiation pattern and a relative target to munition speed of say 3200 feet per second, 20 mm. shell label of 60X 10-6 curies of gamma activity from antimony 124 will give an `accuracy of i2S% in scoring indication of the threshold hit distance, using some form of background noise suppression device. At 30 -feet 240 106 euries `of :activity produces approximately i20% accuracy of the threshold hit distance. Using twice the foregoing activity, -the accuracy can be improved to 1*- 20% at 15 feet and about il7% at 30 Ifeet. To obtain il0% .accuracy a label of 3 millicuries per shell is necessary. Both distances of l5 and 30 feet can be simultaneously scored by using the activity for the greater distance. Depending on the quantity of shells and the number labeled per mission it is reasonable to use an activity of up to about 10 millicuries per shell.

With larger air-to-air missiles a uniform target volume is defined for relative speeds up to 2,000 feet per second with a radioactive label as small as microcuries to mi Y.

1 1 i20% accuracy at 15 feet and 30 feet. With 50 microeuries the accunacy is 125 The laccuracy of score improves to il% at l5 by the use of 2.5 millicuries. Ten millicuries of activity will give an accuracy of i 10% at 30 feet. It is practical to use up to 25 millicuries with large air-to-air missiles, because relatively few are required.

ANOTHER EMBODIMENT FIGURE 2 illustrates a form of the invention which may be used to determine the flight path of a missile with respect to an airborne target. In this form the target comprises a sleeve 50 of conventional type but preferably smaller in size than the conventional sleeve, and a pair of radiation detector and radio transmitter units 52 and 54 which are arranged at opposite sides of the sleeve 50. The various parts of the target are attached to a tow line 57 which is pulled by an aircraft 59. The effective target volume is determined inter alia by the spacing of urn'ts 52 and 54. Tlhe sleeve 50 forms the visible target center. By proper use of miniaturized, lightweight components, the entire target assembly including units 50, 52, and 54 will weigh no more than the conventional sleeve target. Although units 52 and 54 are shown in block form, the practice these units will have an aerodynamically designed housing so that the drag will be minimized.

The same basic target construction can be used for ranges from 200 to about 2,500 feet by changing only the spacing between the units 52 and S4 on the tow line from 50 to 1,000 feet. The strength of the nuclear source on the missile 10 is selected to tit the type of missile and the effective :target size. The design of radiation source and detector may be based on an accuracy of miss-distance measurement of to 10% of the receiver spacing near the target center and of the maximum range near the periphery.

Units 52 and 54 are preferably self-powered units inoluding a radiation `detector head 56 or 581, an amplilier, and a small telemetering transmitter. The power pack can be made very small if `the units are energized by a remote signal for the very short time that the target is under attack. Alternatively, an air driven generator could be used for the power supply.

In order to obtain an accurate plot of the path of the missile relative to the target, each detector head is made directional. In FIGURE 9, the respective heads are shown at 56 and 58, each including a sphere of scintillation material 60, a plurality of photomultiplier tubes 62, lead divider plates 64, and a light-tight housing 66. In the form shown the divider plates 64 separate each detector into quadrants, each quadrant of scintillation material having its own photomultiplier tube in contact therewith. As previously described, the scintillation material may be coated with suitable reecting substances. With radiation detectors of the type shown, overlapping directional radiation patterns are produced.

The outputs of the respective detector head sections are transmitted from units 52 and 54 to a radio receiver and flight path computer unit 68 which may be located on the ground. The outputs of the various detector sections will vary with the radiation received and hence with the range and direction of the missile with respect to the target. Since the spacing between the units 52 and 54 is known, the flight path of the missile with respect to the target may be readily computed from the relative outputs of the respective detector head sections. The radioactive data may be supplemented by the known ballistic properties of the missile and the altitude and velocity of the target. The computed flight path of the missile may be presented as a two dimensional graphic display in a plane of Hight. The third dimension rnay be determined by the angular coordinates of this plane of missile flight with respect to the target path. From the computed flight path of the missile relative to the path of the target, the miss-distance may be readily determined.

In the system of FIGURE 2, the counting rate data 12 transmitted is essentially continuous, rather than go, rio-go as in the system of FIGURE 1. Any standard telemetering system may be employed for the transmission of such data. For example, frequency division or time division multiplex techniques may be used.

The following tables give two practical examples of cases in which the invention illustrated in lFIGURE 2 may be employed. In Case 1, the data concerns a small missile such as a mm. shell, while in Case 2 a large missile is assumed.

Case 1 Projectile: 90 mm. shell. Target region: Sphere, 600 feet diameter. Nuclear radiation: Choice of Sodium 24, Cobalt 60 and others. Strength: 10 millicuries (Symmetrical radiation pattern). Radiation field: 13 milliroentgens per hour at 1 meter.

With no shielding-1.3 milliroentgens per hour at y10 feet. Target receiver: 300 to 1,000 cm.2 sensitive receiving area; choice of reception pattern depending on requirements. Receiver spacing: 200 feet on tow line. Total receiver weight (in aerodynamic housing): 10-25 pounds depending upon design. Other design features:

(a) Signal rate at target when projectile is at maximum range: 2,000 counts per second. (b) Signal nate at center: 20,000 c.p.s. (c) `Projectile closing speed: 3,000 f.p.S. (d) Duration of signal: 1/5 second. Accuracy:

i4 feet at center. i30 feet at periphery.

Case 2 Projectile: Missile.

Target region: Sphere l mile in diameter.

Radiation emitter properties:

Cobalt 60.

10 euries.

0.1 cubic inch volume. Weight less than 1 ounce.

Radiation Field: Since no personnel are in the vicinity of the in-flight missile, a strong source can be used with no shielding. For installation and maintenance appropriate shields can be easily used. (The source placed in 1an S50-pound spherical lead or 45-pound tungsten shield is -nonhazardous for storage and handling.)

Receiver: 300 to 1,000 cm.2 etfective sensitive receiving tarea of veach receiver; choice of reception pattern depending upon requirements.

Receiver spacing: 1,000 feet.

Total receiving weight: 10-25 pounds with 'aerodynamic housing.

Other design features:

(a) Signal rate at maximum range: 200 counts per second.

(b) Signal rate at center: 40,000 counts per second.

(c) Projectile speed: 1,000 feet per second.

(d) Duration of signal: 5-seconds.

'I 'he following table gives representative miss-distance deslgn parameters. In this table it is assumed that two radiation detectors `are spaced on a tow line, each detector with a 1,000 square centimeter effective area.

From the foregoing description of the invention it is apparent that unique missile scoring systems are provided. While preferred embodiments of the invention have been shown and described, it will be apparent to those skilled in the art that changes can be made without departing from the principles and spirit of the invention, the scope of which is defined in the yappended claims. Accordingly, the foregoing embodiments are to be considered illustrative, rather than restrictive of the invention, and those modifications which come within the meaning and range of equivalency of the claims are to be included therein.

The invention claimed is:

1. A system `for scoring hits of missiles upon a target, the missiles being provided with a source of nuclear radiation, said system comprising a nuclear radiation detector at said target of the type which produces a number of pulses in `a predetermined time as a function of the distance from the radiation source, a hit indicator, and means coupling said radiation detector and said hit indicator for causing said hit indicator to respond to radiation detected by said detector only upon the production from said detector of at least a predetermined number of pulses in a predetermined length of time, corresponding to a predetermined radiation threshold level.

2. The invention of claim ll, said coupling means comprising an integrator for integrating the output of said radiation detector.

3. The invention of claim 1, ysaid coupling means comprising a pulse height selector.

4. The invention of claim 1, said target being remote from said hit indicator, and said coupling means comprising a telemeter transmitter.

5. The invention of claim 4, said transmitter having a source of carrier waves, and said coupling means comprising means responsive to detected radiation for modulating said carrier waves.

6. The invention of claim l, said coupling means having automatic calibration means for maintaining said threshold level.

7. The invention of claim 1, wherein said hit indicator is visual.

8. A system for scoring hits of missiles on a target, the missiles having a source of nuclear radiation, said system comprising a nuclear radiation detector at said target of the type which produces pulses in response to detected radiation, a hit indicator, and means coupling said detector and said indicator for causing said indicator to indicate a hit vonly upon the detection by said detector of nuclear radiation from said missiles above a predetermined threshold level, said coupling means comprising a first pulse height selector for passing pulses of a first magnitude, and calibration means for maintaining said threshold level, said calibration means comprising a calibration source of nuclear radiation to which -sai-d radiation detector is exposed, the radiation from said calibration source being of a magnitude `diterent from the magnitude of the radiation received from said missiles, said calibration means having a second pulse height selector responsive to pulses corresponding to the radiation received from said calibration source.

9. The invention of claim 8, said system comprising an amplifier coupled to the output of `said radiation detector, and said calibration means comprising means for varying the gain of said amplifier.

10. A system for scoring hits of missiles within two different r-anges from a target, the missiles being provided with a source of nuclear radiation, said system comprising a nuclear radiation detector at the target of the type which produces pulses in response to detected radiation, indicator means for indicating hits within the respective ranges, and means including a pair of integrators coupling said indicator means and said detector, said integrators having different time constants corresponding to different pulse rates produced by said detector in response 1.4` to radiation received from said missiles withinthe respective ranges.

11. The invention of claim 10, said coupling means comprising a radio transmitter connected to said integrators, said indicator means including a radio receiver, and means for modulating said transmitter dilerently in response to the outputs of the respective integrators.

12. The invention of claim- 11, said modulating means comprising -a variable frequency oscillator, the frequency of said oscillator being controlled by the outputs of said integrators.

13. The invention of claim 12, further comprising automatic calibration means for said system, said calibration means comprising additional means for controlling the frequency of said oscillator.

14. A system for scoring hits of missiles upon an airborne target, the missiles being provided with a source of nuclear radiation, said target having an omnidrectional nuclear radiation detector of the type which produces a number of pulses in a predetermined time as a lfunction of the distance from the radiation source, and having a transmitter for transmitting signals representative of hits upon said target to a remote receiver and hit indicator, and means coupling said detector and said transmitter for causing said transmitter to transmit said signals only upon the production from said detector of at least a predetermined number of pulses in a predetermined length of time, corresponding to a predetermined radiation threshold level.

15. The invention 'of claim 14, further comprising a visual hit indicator at said target actuated from said coupling means.

16. The invention of claim 1, said radiation detector having a plurality of radiation responsive parts, said coupling means having means for lactuating said hit indicator only upon the detection of radiation by a single part.

17. The invention of claim l, said target having a detector responsive to energy other than said nuclear radiation and means for preventing the actuation of said hit indicator except when said radiation detector and said energy detector produce concurrent outputs.

18. The invention of claim 17, said energy detector comprising a shock wave detector.

19. The invention of claim- 17, said energy detector comprising a radio wave detector.

20. In a system for determining the path of a missile relative to a target, the missile being provided with a source of nuclear radiation; a pair of spaced nuclear radiation detectors, each of lsaid detectors having a directional radiation pattern, the radiation patterns of said detectors overlapping, and computer means coupled to said detectors for producinng a two-dimensional iiight path output as a function of the relative outputs of said dete'ctors.

2l. A system for scoring hits of missiles within two diierent ranges from a target, the missiles being provided with a source of nuclear radiation, said system comprising a nuclear radiation detector at the target having an output which is va function of the distance from the radiation source, means for indicating hits within the respective ranges, and means responsive to the output of said detector for producing a hit indication for one range when the output exceeds a first threshold and for producing a hit indication for the other range when said output exceeds another threshold.

22. A system for scoring hits of missiles upon a target, the missiles being provided with a source of nuclear radiation, said system comprising a nuclear radiation detector at said target, a hit indicator, and time constant means :responsive to the output of said detector for actuating said indicator only when the output of said detector has a predetermined value Within -a predetermined length of time set by said time constant means.

(References on following page) 15 16 References Cited in the le of this patent 2,535,255 Barnes e Dec. 26, 1950 2,616,967 Beukema, Nov. 4, 1952 UNITED STATES PATENTS 2,628,836 Gangel Feb. 17, 1953 2,414,479 Miller Jan. 21, 1947 2,795,778 Bagby June 11, 1957 2,448,587 Green Sept. 7, 1948 5 2,890,334 Gille June 9, 1959 HNI 

1. A SYSTEM FOR SCORING HITS OF MISSILES UPON A TARGET, THE MISSILES BEING PROVIDED WITH A SOURCE OF NUCLEAR RADIATION, SAID SYSTEM COMPRISING A NUCLEAR RADIATION DETECTOR AT SAID TARGET OF THE TYPE WHICH PRODUCES A NUMBER OF PULSES IN A PREDETERMINED TIME AS A FUNCTION OF THE DISTANCE FROM THE RADIATION SOURCE, A HIT INDICATOR, AND MEANS COUPLING SAID RADIATION DETECTOR AND SAID HIT INDICATOR FOR CAUSING SAID HIT INDICATOR TO RESPOND TO RADIATION DETECTED BY SAID DETECTOR ONLY UPON THE PRODUCTION FROM SAID DETECTOR OF AT LEAST A PREDETERMINED NUMBER OF PULSES IN A PREDETERMINED LENGTH OF TIME, CORRESPONDING TO A PREDETERMINED RADIATION THRESHOLD LEVEL. 